The long-term objective of the proposed project is to understand the mechanism of load-driven bone lengthening. Although distraction osteogenesis is effective to treat the patients with limb length discrepancy, this invasive procedure occasionally generates problems such as premature consolidation, delayed union, and infection. In order to investigate a possibility of load-driven non-invasive therapy, we will focus on knee loading a form of joint loading modalities. In this R03 project, we address a set of questions: (1) Does knee loading enhance proliferation of chondrocytes in the growth plate and lengthen the proliferative and hypertrophic zones in the distal femur and the proximal tibia? (2) Is the lengthening effect dependent on ages? (3) Does injection of insulin growth factor-2 (IGF-2) into the growth plate enhance the loading effects? We hypothesize: (a) Lateral loads, applied to the knee with appropriate loading conditions, can lengthen both the femur and the tibia through stimulating proliferation of chondrocytes in the growth plate;(b) Efficacy of load-driven bone lengthening is stronger in the youth than the elderly;and (c) The lengthening effect can be augmented in the elderly by injecting IGF-1 in the growth plate. In order to examine the above hypotheses, three specific aims are proposed using a hindlimb of C57BL/6 mice as a model system. " Evaluation of load-driven bone lengthening under varying mechanical conditions " Comparison of the loading effects among mice with varying ages " Effects of local administration of IGF-2 into the growth plate with and without knee loading. We will use a custom-made piezoelectric mechanical loader, and conduct bone histomorphometric and gene expression analyses using varying imaging modalities. The proposed study is expected to contribute to developing a non-invasive physical therapy for treatment of patients with limb length discrepancy. PROJECT NARRATIVE Currently the invasive surgical procedure (distraction osteogenesis) is commonly employed to lengthen long bones. However, this procedure presents a series of potential risks such as premature consolidation, delayed union, bone infection, joint stiffness, and fracture of newly formed bone. The proposed R03 project will contribute to examining a possibility of non-invasive physical therapy for treatment of patients with limb length discrepancy.